The present invention relates generally to apparatus and methods for providing constant voltage from a permanent magnet generator at varying prime mover speeds or under varying loads.
A conventional salient pole synchronous machine, or wound rotor machine, can provide constant voltage output, but without a gear-reduction stage it is incompatible with high-speed prime movers. Further, this type of machine has poor reliability and low efficiency. In contrast, the rotor construction of permanent magnet generators is robust and allows for high-speed operation, but excitation must be fixed unless a full-scale three phase switching bridge or active power electronics with similar complexity are used to condition the variable voltage produced by a permanent magnet generator. Three phase switching bridges are very expensive for high power applications. They also present reduced reliability and increased volume and weight. The losses in the switching devices are also substantial.
Prior art patents such as U.S. Pat. Nos. 4,755,736 and 4,223,263 describe a main generator and a second permanent magnet generator/exciter. The main generator has a single ac winding positioned on a stator and a single dc winding positioned on a rotor. The dc rotor winding receives excitation from the permanent magnet generator/exciter mounted on the rotor. The generator output voltage is controlled by the magnitude of dc current present in the dc rotor winding. A mechanical source of power is required to drive the rotor.
U.S. Pat. No. 5,955,809 describes a dual axial gap permanent magnet generator that requires two inversions, from ac to dc and from dc to ac, to control the voltage output.
As can be seen, there is a need for producing voltage in a relatively narrow controlled range. At the same time, the prime mover should operate at variable speed in order to have fuel efficient performance.